1. Field of the Invention
The present invention relates to an optical recording medium and a method of manufacturing such an optical recording medium.
2. Description of the Related Art
There have been known optical recording mediums such as optical disks, cards, etc. for recording and/or information such as audio information, video information, etc. with the application of light. It has been proposed to increase the amount of information that can be recorded on an optical recording medium by employing an information recording surface thereof which is of a multilayer structure.
One such proposal is illustrated in FIG. 1 of the accompanying drawings. As shown in FIG. 1, an optical disk has a pair of first and second recording members 1, 2 which comprises respective transparent substrates 1S, 2S, respective information areas 1M, 2M disposed on respective surfaces of the transparent substrates 1S, 2S and carrying recorded information in the form of information pits, and respective reflecting surfaces 1R, 2R of evaporated aluminum, for example, disposed respectively on the information areas 1M, 2M. The first and second recording members 1, 2 are bonded to each other by an adhesive 3 with the information areas 1M, 2M facing each other.
For reading or reproducing recorded information from the information areas 1M, 2M, reading light (hereinafter referred to as reproducing light) 1L is applied to the information area 1M through the transparent substrate 1S, and reproducing light 2L is applied to the information area 2M through the transparent substrate 2S. Therefore, a drive unit for use with the optical disk has a double optical pickup structure including two playback optical pickups positioned in confronting relationship to the opposite surfaces of the optical recording medium for applying reproducing light to the respective information areas 1M, 2M, or alternatively, a single optical pickup for applying reproducing light to the information areas 1M, 2M which have selectively been brought into facing relationship to the optical pickup by reversing the optical disk.
The drive unit of the double optical pickup structure is large in size, made up of a large number of parts, and heavy in weight. The drive unit with the single optical pickup needs a complex mechanism for automatically reversing the optical disk in order to successively reproduce recorded information from the information areas 1M, 2M. Therefore, the drive unit with the single optical pickup is also large in size, made up of a large number of parts, and heavy in weight.
To avoid the above difficulties, there has been proposed an optical recording medium capable of reproducing information recorded on two-layered information recording areas with light applied from one side of the optical recording medium. The optical recording medium and a method of manufacturing same will be described below with reference to FIGS. 2A through 2D of the accompanying drawings.
As shown in FIG. 2A, a first transparent substrate 21S is injection-molded of polycarbonate (hereinafter referred to as "PC") which has a first information area 21M comprising information pits representative of recorded information. Then, as shown in FIG. 2B, a semitransparent optical layer 23, for example, a sputtered layer of SiN, is formed on the first information area 21M for controlling the transmittance and reflectance in the first information area 1M at desired values. Thereafter, a second information area is formed of an ultraviolet-curing resin on the semitransparent optical layer 23 by a photopolymerization process, e.g., a so-called 2P process. More specifically, as shown in FIG. 2C, a stamper 25 having a pattern of convex and concave shapes for forming a second information area is pressed against the semitransparent optical layer 23 through an ultraviolet-curing resin 24, and then the ultraviolet-curing resin 24 is cured by an ultraviolet radiation applied from the side of the first transparent substrate 21S. Thereafter, as shown in FIG. 2D, the stamper 25 is removed, leaving a second information area 22M, transferred from the pattern of convex and concave shapes of the stamper 25, on the ultraviolet-curing resin 24. A reflecting film 26 of evaporated aluminum or the like is deposited on the second information area 22M, and then covered with a protective layer 27 of a photosetting resin or the like. In this manner, the optical recording medium with the first and second information areas 21M, 22M is manufactured.
For reproducing information recorded on the optical recording medium with light, the light is applied to the optical recording medium from the side of the transparent substrate 21S. More specifically, as shown in FIG. 2D, to reproduce recorded information from the first information area 21M, reproducing light 21L which is focused on the first information area 21M is applied from the side of the transparent substrate 21S, and light reflected by the first information area 21M is detected to read the information. To reproduce recorded information from the second information area 22M, reproducing light 22L which is focused on the second information area 22M is applied from the side of the transparent substrate 21S, and light reflected by the second information area 22M is detected to read the information.
Since the information recorded on the first and second information areas 21M, 22M can be reproduced with light applied from one side, a drive unit for use with the optical recording medium may have a single optical pickup. As the optical recording medium does not need to be reversed, the drive unit may be small in size, light in weight, simple in structure, and easy to assemble. However, because the information area 22M is formed by the 2P process, the optical recording medium is required to be formed in layers, and cannot be mass-produced efficiently.